Flywheels have been used for many years as energy storage devices. They have often been used as power smoothing mechanisms for internal combustion engines and other kinds of power equipment. More recently, flywheels have been recognized as a very attractive energy storage technology for such electrical applications as uninterruptible power supplies, utility load leveling systems, electric vehicles and for storage with alternative energy generation.
Modern flywheel energy storage systems convert back and forth between a spinning flywheel""s rotational energy and electrical energy. A flywheel energy storage system includes a flywheel; a motor generator, a bearing system and a vacuum enclosure. The rotating flywheel stores mechanical energy, the motor generator converts electrical energy to mechanical energy and visa-versa, and the bearing system physically supports the rotating flywheel.
To compete with lower cost electrochemical batteries, the flywheel systems must maximize their possible energy storage. To do this, the flywheel speed is preferably maintained at about the full charge speed and the full charge speed must be set as high as safely allowable by the flywheel structural capability. This maximizes the flywheel operating stress and hence the energy storage capability. When operating at high stress levels, accurate and reliable speed control is essential for safety, and for this reason speed control is a critical issue.
Flywheel systems typically employ brushless type synchronous motor/generators for long life and hence electronics are required to provide the commutation. One way possible to drive the motor/generator of a flywheel system is to use a servo amplifier or pulse modulated bi-directional inverter. The servo amplifier converts power to synchronous AC to drive the motor/generator coils and a separate reference input voltage is used to control the duty cycle of the pulses supplying power through pulse modulation. In some cases, the fluctuations in line voltage that powers the motor can cause the flywheel to have less than a full charge or likewise cause the flywheel to accelerate to higher than safe speeds. This type of problem is possible when the flywheel motor/generator is driven using a servo amplifier operating in simple duty cycle (open loop) mode. Derating the flywheel to account for input charging voltage deviations results in less than full energy storage for the flywheel system, an undesirable result. In telecommunications backup applications, the flywheel specification restricts speed deviations to no more than 2% above the rated speed.
To circumvent this problem, the flywheel speed can be kept at maximum speed regardless of fluctuations in the charging input voltage by operating a servo amplifier in an alternative mode such as a velocity mode. Velocity modes works using a velocity Proportional voltage signal as feedback that adjusts the duty cycle. The signal can come from either a tachometer or from a frequency-to-Voltage conversion circuit with position sensor input. The speed control can be conducted inside the servo amplifier itself or with the use of circuits. Unfortunately, these methods are not desirable or reliable for very long term operation due to several deficiencies. In operation, a summing amplifier generates an error signal that is proportional to the difference between the desired speed and the actual speed. This analog error signal is then used to set the duty cycle of the servo amplifier duty cycle. When the flywheel is accelerated, the output duty cycle may be initially limited by the current control loop in the amplifier at low speeds. The flywheel will initially accelerate at maximum current as desired. However, near the maximum speed the error signal on the speed control becomes smaller and smaller thus reducing the duty cycle and acceleration when close to full speed. The flywheel system can suffer by taking an exceedingly long time to fully charge. Other problems also arise related to the analog nature of the feedback loop. The setting of the operating speed requires a manual adjustment to set the exact required amplifier gain. This is both subject to error and can be difficult to test in flywheel systems that take hours to fully charge. These analog circuits are also subject to changing performance over time due to amplifier and component drift and degradation as well as with changes in temperature. Regardless of whether a flywheel motor/generator is driven by a servo amplifier or by another means, an accurate and reliable speed control method is needed, which should also preferably be low in cost.
Accordingly, the invention provides a speed control for a flywheel energy storage system that provides accurate and reliable speed control for long-term operation. The speed control uses a current limiter to safely limit the acceleration current to the motor for accelerating the flywheel, and a rate controller that digitally switches the acceleration current on and off to maintain the desired steady state speed. The rate controller turns the acceleration current off when the flywheel speed reaches a predetermined steady state speed and then turns the acceleration current back on when the flywheel speed falls below the desired steady state speed. Use of such an simple and economical speed control for controlling the speed of brushless motors is unconventional and against the well established principles in the art of motor control. Using an on/off speed control in conventional applications would result in very wide oscillations in speed with considerable overshoots and undershoots. The undesirable oscillations would also fatigue any attached rotating structures over long-term operation.
However, flywheel energy storage systems are unique applications for brushless motors and this speed control method of the invention is well suited to use in flywheel systems, in part because the flywheel unusually has a large rotational inertia comparatively to the power level of its motor/generator. Flywheel systems also typically operate at high speeds, and because the power level is proportional to the torque multiplied by the rotational speed, torque levels can also be reduced at high operating speeds. Both of these attributes can cause the flywheel speed to, only be able to change relatively slowly when full torque is applied. The effect is that the large inertia of the flywheel sufficiently damps out any wide oscillations that would occur in the flywheel speed caused by the digital type control.
The digital type speed control can be made both more accurate and reliable by using straightforward on/off control about the desired steady state operating speed. The speed control is easy to implement, low in cost and is not subject to change over long-term operation. The speed control also charges the flywheel system as rapidly as possible by accelerating the flywheel with the maximum charging current of the rotor up until full operating speed is reached. Application of the speed control method to both (low power to energy storage) flywheel systems and (high power to energy storage) flywheel systems provides satisfactorily tight control. Calculations based on using a very slow on/off switching rate of only 1 Hz, low power and high power flywheel systems show speed oscillations of less than 0.01% and 0.7%, respectively. Considering that stress in a flywheel is proportional to the square of its speed, these speed oscillations in both cases cause stress oscillations that are less than 1.4%, rendering any fatigue effects on the flywheel completely insignificant. Use of a faster switching rate reduces oscillation amplitude even further.
In one embodiment, the rate controller operates by measuring the frequency of excitation of a rotational position sensor and turns the acceleration on or off based on whether the frequency of excitation exceeds a threshold value corresponding to the desired steady state speed, or is above or below a predetermined range of such values.
In another embodiment, the rate controller operates by measuring the average voltage across one or more electromagnetic coils that are magnetically excited by the rotation of the flywheel and turns the acceleration on or off based on whether the average voltage exceeds a threshold value corresponding to the desired steady state speed, or is above or below a predetermined range of such values. In a further embodiment, this method is used and the electromagnetic coils are phases of the armature coils of the motor/generator. This method can be used to control steady state operating speed in very low cost flywheel systems or alternatively can be used as a safety over-speed prevention method whereby the elevated average voltage, disconnects the charging power.